1. Field of the Invention
This invention relates to a new aluminum base copper-lithium-magnesium alloy. More particularly, it relates to such an alloy for use in welding aluminum lithium alloy products together and which is characterized by improved resistance to weld cracking, high weld strength, and good corrosion resistance in the weld zone.
2. Description of the Related Art
Within the past few years development of aluminum lithium alloy products has received considerable attention for aerospace applications. Aluminum lithium alloy products have mechanical properties comparable to products made from typical aircraft alloys but with a 5-11% lower density. Use of such alloys, therefore, offers weight savings which can either reduce the cost of operating an aircraft or can permit an increase in the payload that the aircraft can carry. Aircraft are typically assembled with mechanical fasteners but with increased commercialization of aluminum lithium alloys for aircraft, such alloys will likely be considered for applications where joining by welding may be preferred. Such use may include, for example, cryogenic uses, marine hardware, lightweight pressure vessels, or as armor plate.
It is known that aluminum lithium alloys can be welded. A report dated November 1984 from Martin Marietta Laboratories entitled "The Weldability of Al-5Mg-2Li-0.1Zr alloy 01420" by J. R. Pickens, T. J. Langan, and E. Barta reports on work performed by Martin Marietta to assess weldability of lithium-containing aluminum alloys. The authors cited a number of Soviet publications concerned with aluminum lithium alloys and/or welding thereof. They reported that the most widely used aluminum lithium alloy in the world is Soviet developed alloy 01420. The nominal composition of this alloy was given as 5 wt. % Mg and 2 wt. % Li. Variants of the alloy may also contain either 0.05-0.3 wt. % Zr, 0.2-1.0 wt. % Mn, 0.05-0.3 wt. % Cr, 0.05-0.15 wt. % Ti, or 0.05-0.3 wt. % Cr+&lt;0.1 wt. % Ti for grain refinement.
The test work performed and reported in the Pickens et al publication was done on joining an 01420 alloy having 0.075 wt. % Zr as a grain refiner. Test welds were made using the TIG method of welding and using either 01420 parent metal as filler or Aluminum Association (AA) alloy 5356 having the composition 5 wt. % Mg, 0.4 wt. % Fe, 0.25 wt. % Si, 0.2 wt. % Mn, 0.2 wt. % Ti, 0.2 wt. % Cr, and 0.1 wt. % Zn with the balance consisting of aluminum. From tests performed, the authors concluded that the 01420 aluminum lithium alloy was weldable with weld characteristics varying with the filler alloy, preparation of the weld zone prior to welding, and thermal treatment of the weld zone after welding.
An aluminum lithium alloy developed in the United States and recently designated as Aluminum Association alloy 2090 is designed for high strength applications in the T8 temper. In addition to lithium, AA alloy 2090 is an aluminum base alloy which includes copper as a principal alloying element and has a nominal composition of 2.2 wt. % Li, 2.7 wt. % Cu, and 0.12 wt. % Zr. It is anticipated that in the United States the use of aluminum base copper-lithium alloys such as AA alloy 2090 will grow, and it would be desirable to provide a filler alloy for use in welding such alloys which will produce optimal joint characteristics.
Such optimal joint characteristics would include insensitivity to weld cracking, good resistance to weld corrosion, and the ability to develop high weld strength during subsequent aging.